1. Field of the Invention
The present invention concerns a travelling wave tube provided with an impervious coupling device between its delay line and an external microwave circuit.
The invention can be applied, particularly, to wide-band power travelling wave tubes, the delay line of which has the shape of a helix structure or a shape derived from a helix.
2. Description of the Prior Art
A travelling wave tube of this type essentially comprises:
an electron gun designed to emit an electron beam in an essentially rectilinear direction; PA1 a delay line placed downstream of the electron gun and enclosed in a cylindrical sheath which is concentric with it. The electron beam is emitted in the extension of the longitudinal axis of the delay line, which it crosses from end to end. It is therefore in the sheath of the delay line that the interaction occurs between the electron beam and a microwave guided by the delay line. The delay line is provided with an injection device and with a device for the extraction of microwave energy; PA1 a permanent-magnet focusing structure, concentric with the sheath of the delay line and extending over the entire length of the delay line. This focusing structure creates a magnetic field in the sheath of the delay line, and this magnetic field has the effect of keeping the paths of the electrons of the electron beam rectilinear; PA1 a collector designed to collect the electron beam downstream of the delay line.
The travelling wave tube is generally connected to external microwave circuits, on the incoming side of the delay line as well as on its outgoing side, by means of a linking element forming one end of a transmission line of these circuits. On the incoming side of the delay line, this linking element, through which the microwave energy is injected, generally has the structure of a coaxial line, since the level of energy to be injected is low. On the outgoing side of the delay line, this linking element, through which the amplified microwave energy is extracted, has the structure of either a delay line or a ridged wave guide, depending on the level of the energy to be extracted.
In a known way, the electromagnetic coupling between a connecting element and an end of the delay line is done by means of a coupling pin having one end brazed to the corresponding end of the helix forming the delay line, and having a coupling surface located so that it faces an end surface of the central conductor or of the ridge of the linking element, with no mechanical contact being set up between the coupling pin and the linking element.
The different elements of the travelling wave tube and the linking elements form a set with communicating cavities. The parts of this set are assembled imperviously, and the set is itself sealed hermetically so that the high vacuum (10.sup.-8 to 10.sup.-9 Torr) needed for the working of the electron gun can be created in its cavities. That end of the linking elements which is opposite the travelling wave tube is thus blocked by a window, called a microwave window, which is permeable to the microwave energy and impermeable to the gases.
This arrangement of a travelling wave tube and of the linking elements to the external circuits, which forms part of the prior art, has a major drawback.
For, the volume of gas to be pumped in all the cavities of the travelling wave tube and of the linking elements is generally far greater than the volume of the cavities where it is functionally necessary to create a vacuum, namely, the cavities through which the electrons travel. In fact, it is not rare for the internal volume of a linking element to be of the same magnitude as the internal volume of the travelling wave tube itself. Since the pumping is most usually done at the sheath of the delay line, the internal dimensions of which inasmuch as it demarcates the interactive zone of the tube, are inversely proportionate to the working frequency of the tube, the pumping time for the high frequency tubes reaches values that are incompatible with the requirements of industrial-scale production. Furthermore, the vacuum obtained is not always as high as would be desirable.